This invention relates to a bridge circuit composed of semiconductor strain gauges, and more particularly to a semiconductor strain gauge bridge circuit device including a zero-point temperature compensation circuit for compensating a temperature-dependent drift of a zero-point output of the bridge circuit.
Pressure transducers including a bridge circuit of semiconductor strain gauges for transducing a pressure utilizing the piezoresistive effect of a semiconductor are already widely known and used. However, because the resistance value and piezoresistance coefficient of the semiconductor forming the strain gauges are very sensitive to a temperature change, the semiconductor strain gauges are defective in that their output sensitivity and zero points are greatly adversely affected by the temperature change. Therefore, suitable temperature compensation means is required for detecting an exact amount of strain of the semiconductor strain gauges.
A zero-point temperature compensation circuit is known in which fixed resistors are connected in series and parallel with one of semiconductor strain gauges incorporated in adjacent two branches of a bridge circuit, so that the temperature-resistance characteristic of that semiconductor strain gauge coincides with that of the other semiconductor strain gauge. However, this arrangement is difficult to employ because complex calculations are required to determine the resistance values of these fixed resistors. Further, the prior art arrangement has been defective in that, because one of the fixed resistors is connected in series with one of the semiconductor strain gauges in the bridge circuit, many connection points between the bridge circuit and associated circuits are required. Many, for example, five or more connection terminals have been required which include a pair of power supply terminals of the bridge, a pair of output terminals of the bridge, and one of the terminals of the semiconductor strain gauge to which the series resistor is connected.
Such a prior art problem is solved by an invention disclosed in, for example, U.S. Pat. No. 4,480,478 issued to Sato et al on Nov. 6, 1984. According to the disclosure of the invention described above, there is provided a circuit for generating a voltage equal to a midpoint voltage appearing at a middle point (an output terminal) of a semiconductor strain gauge bridge circuit, at a predetermined temperature, and this voltage is connected through a resistor to one of the output terminals of the bridge circuit. That is, the zero-point temperature compensation of the bridge circuit of the semiconductor strain gauges is attained by adjusting so that the zero-point output of the bridge circuit at a first predetermined temperature t.sub.1, such as the room temperature, and the zero-point output at a second predetermined temperature t.sub.2 higher than the first predetermined temperature t.sub.1 are equal to each other.
However, although the desired zero-point temperature compensation at temperatures between the first predetermined temperature t.sub.1 and the second predetermined temperature t.sub.2 can be satisfactorily made according to the prior art disclosure, the prior invention disclosed in the Sato patent has the drawback that sufficient temperature compensation cannot be attained at other temperatures, that is, in a temperature range lower than t.sub.1 or higher than t.sub.2. Referring to JP-A-58-140604 regarding this drawback, the zero-point temperature compensation is not done by changing the shape of the temperature characteristic curve of the bridge circuit, but, instead, the characteristic curve is rotated around a reference point V.sub.01 to widen the flat portion of the characteristic curve as seen in FIG. 4 of the publication. Thus, when the temperature characteristic of the bridge circuit itself includes a nonlinear curvature occurring outside of the predetermined temperature range described above, this nonlinear curvature of the temperature characteristic cannot be compensated. As a result, the zero-point output of the bridge will undergo a nonlinear drift relative to a temperature change, and it is unable to attain the desired zero-point temperature compensation with high accuracy over a wide temperature range.